Among the most widely documented findings in the psychological literature is that mental processing speed slows with advancing age. Most of the studies done evaluating this change have used one measure of processing speed, reaction time, both to assess the degree of slowing and to infer the extent to which it affects all components of mental processing or spares some while influencing others. Meta- analyses assessing the results of aggregated reaction time studies suggest that all processing components are slowed. The first models to evolve from this work postulated a decline that affected all elements of processing indiscriminantly. Recent evolutes of these models likewise conceive of the decline as generalized, but characterize the slowing as being increasingly pronounced as new processing demands are imposed. Although these models do not explicitly hypothesize a pattern of differential slowing, they can be interpreted to implicate the response end of processing more than the stimulus end. A meta-analysis in which the latency of an event- related brain potential (ERP), the P300, and reaction time were evaluated revealed a different pattern of effects for the two measures, suggesting that aging does produce a generalized slowing but that response processing is slowed more than is stimulus processing. Support for this conclusion is found in other studies that have use P300 latency and reaction time to assess age-related mental slowing, as well as in neurobiological and neuropsychological research that has revealed a pattern of preferential decline in the structural and functional integrity of motor systems in the aged brain. It is hypothesized that age produces generalized slowing that is most severe at the stimulus-response translation level of processing. A test of this hypothesis is proposed using an experimental task that systematically varies two experimental factors known to selectively influence stimulus and response processing, stimulus discriminability and spatial compatibility. The extent to which these factor effects vary in older (68 to 75 years old) and young (18 to 25 years old) subjects will be assessed using multiple dependent measures: reaction time and the latencies of components of the ERP that are differentially responsive to stimulus and response factors. Subjects will be required to respond to stimululation delivered in the somatosensory modality thereby capitalizing on the neuroanatomical and electrophysiological properties of this system to permit reasonably strong inferences to be drawn about the neurocognitive components of the slowing.